Cedric the Entertainer is one of the most versatile comedians around, with the unique ability to improvise characters who are obnoxious and endearing at the same time.

So why force him into a straight-man role in a movie that's nearly a scene-for-scene rip-off of "National Lampoon's Summer Vacation" -- where the only substantive change from the original is a reversed travel route? And once the mistake is made to rein in his humor, why surround him with actors who have no comic timing of their own?

These are the questions that plague "Johnson Family Vacation," a comedy with a couple of big laughs but many more missed opportunities. Cedric tries his hardest, and the Entertainer's heroics keep the movie from being truly awful. But poor direction and a script that borrows liberally from the UPN television sit-com playbook probably doomed this project from the start.

Cedric stars as Nate Johnson, an insurance salesman in a Southern California suburb who is about to travel across country with his estranged wife and three children for a family reunion. The movie begins with Johnson in an auto dealership with his son, where he receives a mistake -- a 2004 SUV version of the Family Truckster that Clark Griswald piloted in the original "Vacation."

It's the exact same beginning as that 1983 movie, minus the humor. The reserved Johnson makes a sort-of-funny comment about his desire to install an eight-track tape player, but mostly expresses sober anger about his new chromed-out set of wheels.

Cedric goes through the movie like a thespian Barry Bonds, with little to no protection in the lineup. As the Johnsons travel from California to Missouri, Cedric is allowed to loosen up a bit, but the supporting players don't give the comic much to work with. The kids who play his children -- including Bow Wow and Solange "Beyonce's sister" Knowles -- seem cast purely for name value.

Movie wife Vanessa Williams is an even bigger comic timing nightmare, and she's had 15 years in the business. It doesn't help that her pairing with Cedric's Johnson is a distracting impossibility. Williams is much too hot for her real-life husband Rick Fox of the Lakers -- does anyone really believe she'd father three children with a boring number cruncher who's built like Tommy Lasorda?

While it's easy to pile on the actors, some of the responsibility must pass to the screenwriters, who failed in the relatively easy task of ripping off a Chevy Chase movie. "Johnson Family Vacation" is mired in seen-it-before nonsense (Shannon Elizabeth is a hitch-hiker who shows a lot of cleavage! And she has a pet alligator that bites Nate in the genitals!) that doesn't even match the ingenuity of Cedric's beer commercials.

Thankfully, the humor picks up when the journey ends at the Johnson family reunion -- a sort of Wally World with sack races and barbecue. Steve Harvey is funnier than usual in a small role as Johnson's brother, and Cedric borrows Eddie Murphy's "The Nutty Professor" trick, donning makeup and overalls to play the Klump-like tow truck driver Uncle Earl.

"Girl, if you don't look like a hot cup of soup," Earl says to Johnson's wife in a backwoods drawl. "And I've got the flu."

Perhaps the makers of "Johnson Family Vacation" should make an entire sequel based on Uncle Earl. (Oh wait, someone already did. It was called "Barbershop 2.") Or maybe next time they should rip off "The Nutty Professor" and let Cedric play the whole Johnson family with the unrestrained hilarity he's known for.